Recently, images are formed on sheets by image forming devices such as an MFP (Multi Function Peripheral). In addition, to reutilize a sheet by removing an image formed on a sheet, the image is printed on the sheet with a color-erasable colorant such as ink containing a leuco dye. The color-erasable colorant can be decolorized under high temperature. In a decolorizing device, a platen roller and a heat source are oppositely arranged via a conveying path of sheets. The sheets are heated when they are conveyed between the platen roller and the heat source.
In addition, the decolorizing device has a scanner to scan the images on the paper before decolorizing and to classify the paper condition after decolorizing. In such a decolorizing device, a circulation path is internally installed to achieve scanning of images, decolorization, and classifying process of sheets. The scanner is disposed downstream of a paper feed unit. A circulation path switching point is installed downstream of the scanner. Moreover, a decolorization unit is disposed in the circulation path, and the circulation path is joined in front of the scanner. A sheet is scanned by the scanner, conveyed to the circulation path, and then decolorized. Next, after the decolorization treatment, the sheet is re-conveyed to the scanner, classified, and conveyed to a paper discharge unit.
However, in case the room temperature is low, the temperature of the decolorizing sheet may be low, or the printing density on the sheet may be high. Sufficient decolorization cannot be realized in any of these cases. For this reason, in a classification process of the sheet after decolorizing, even if the sheet could be completely decolorized under appropriate conditions, the sheet is determined as a non-reusable sheet, lowering the reutilization rate of the sheet.